Electroplating cell including means to agitate the electrolyte in laminar flow

ABSTRACT

An electroplating cell is constructed to prevent current spreading in the electrolyte during the plating of a metal or metal alloy onto a substrate. The cell is constructed such that the cross-sectional area of current path is substantially the same as the cross-sectional area of a pair of electrodes spaced apart in the cell. This is accomplished by placing the electrodes in the cell such that their edges are substantially in contact with the dielectric or insulating walls of the cell. The cell also contains electrolyte agitating means to provide uniform laminar flow of the electrolyte across the surface of one of the electrode. Metal alloy films deposited with the use of this cell exhibit uniform thicknesses on rather large surface areas. Where magnetic metal alloys are plated, the films not only exhibit uniform thicknesses laterally on the whole cathode but uniform composition and magnetic properties throughout as well.

[451 Mar. 28, 1972 54] ELECTROPLATING CELL INCLUDING OTHER PUBLICATIONSMEANS TO AGITATE THE Kronsbein, John; Current and Metal Distribution inElectrodeposition, Plating, Vol 39, No. 2 (Feb. 1952), pp. 165-ELECTROLYTE IN LAMINAR FLOW [72] Inventors: John V. Powers, Shenorock;Lubomyr T.

Romankiw, Millwood, both of N .Y. Primary Examiner john Mack [73]Assignee: International Business Machines Corpora- Assistant Solomantion, Armonk, NY.

Apr. 6, 1970 Attorney-Hanifin and Jancin and Hansel L. McGee ABSTRACT[22] Filed:

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UNITED STATES PATENTS .204/43 5 Claims, 7 Drawing Figures ass-sitesissiugih P'A'TENIEU MAR 28 1972 SHEET 1 [IF 4 INVENTORS JOHN V. POWER-SATTORNEY LUBOHYR T. ROHANKIW FIG.

P'ATE'NTEnmm m2 SHEET 2 OF 4 FIG.2

FIG. 3 (PRIOR ART) PATENTEDMAR28 I972 3. 652,442

sum 3 0F 4 1400 PRIOR ART- CELL PLATED FILM THICKNESS l l 1 1 1 L 1 l 02 4 6 B '10 DISTANCE ALONG THE DIAGONAL OF PLATED FILM (IN CM.)

F|G.5 60k f NI 5ofiT,

Ni DEPOSIT 4o (PRIOR ART CElU COMPOSITION Cu OF Ni Fe, Cu (PRIOR ARTCELL) m y ,4 20 (PRIOR ART cm) 0 A l 1 1 l l 0 l DISTANCE ALONG THEDIAGONAL 0F PLATED FILM (m cm.)

P'ArEmmmzs I872 FIG. 6

CDERCIVE FORCE Ho) 6 ANISTROPY FIELDIHk) I N OERSTEDS DISPERSIONWSO") 4sKEmB) m DEGREES 2 0 n (PRIOR ART can (PRIOR ART CELL) l l I l I I I I II DISTANCE ALONG THE DIAGONAL OF THE PLATED FILMUN CM.)

' (PRIOR ART CELL) I I I I l I I I I 2 4 6 8 DISTANCE ALONG THE DIAGONALOF THE PLATED FILM (IN CM.)

ART CELL) (PRIOR ELECTROPLATING CELL INCLUDING MEANS TO AGITATE THEELECTROLYTE IN LAMINAR FLOW This application is a continuation-in-partof copending U.S. Pat. application Ser. No. 693,375, filed Dec. 26,1967, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an improved electroplating cell, from which metal filmshaving uniform thicknesses and uniform compositions can be deposited.

2. Description of the Prior Art Electroplating, because of its inherentsimplicity, is used as a manufacturing technique for the fabrication ofmetal and metal alloy films. One of the severe problems in plating metalfilms arises from the fact that when a plating current is applied, thecurrent tends to spread in the electrolyte on its path from the anode tothe cathode. This current spreading leads to nonuniform local currentdensity distribution on the cathode. Thus, the film is deposited in anonuniform fashion, i.e., the thickness of the film varies in directproportionality with the current density variation at the cathode.Additionally, where metal alloy films are deposited, for example,magnetic film compositions of nickel and iron or nickel, iron andcopper, this nonuniform current density distribution causes a variationin the compositional makeup of the alloy film. In those cases whereplating is done for decorating purposes, or even in cases of plating forcorrosion protection purposes, the thickness uniformity andcompositional uniformity are not of extreme importance.

When plating is used for the purpose of making thin film electroniccomponents such as resistors, capacitors, conductors, magnetic devicesor other, where both thickness and alloy composition determine theoperation of the device, the uniformity of thickness and alloycomposition are very important and critical. In connection with this,one distinguishes between the variation in composition of the alloythrough the thickness of the film and between the variation ofcomposition and/or thickness from spot to spot laterally over the entireplated area (cathode).

When the films are to be used in computer memories, which demandconstant magnetic characteristics across the entire film compositionalmakeup variation results in deviations of magnetostriction (A). Thisdeviation becomes a severe problem in magnetic films. This is especiallyso in terms of the magnetostriction of the deposited film, since zeromagnetostriction is achieved with alloys including approximately 80percent nickel and percent iron. When the alloy varies by anyconsiderable degree from these proportions, it does not exhibit a zeromagnetostriction. Thus, local composition and thickness of a film arekey factors in determining local magnetic properties of the film. Thefilm properties may vary through the thickness of the film and may alsovary along the lateral profile of the film from point to point over alarge area of the plated film. From the point of view of magneticmemories, it is very important to have the least amount of variation inmagnetic properties, both through the thickness and from place to placeon rather large surface areas.

Considerable progress has been accomplished in the area of obtainingcompositional uniformity through the thickness of the films by thecontrolling of bath makeup and operating conditions. For example, incopending U.S. Pat. application Ser. No. 573,417, filed Aug. 18, 1966,now U.S. Pat. No. 3,480,522, to James M. Brownlow and assigned to thesame assignee as is this application, there is disclosed a method ofcontrolling plating conditions by pulse plating in combination with adilute plating bath.

In contrast to the above solution for obtaining uniformity through thefilms thickness, very little progress has been made in obtaining lateraluniformity in the plated film. In this area, use of aids to improvecurrent distribution has been the main approach to solving the problem.Normally, current balancing has been accomplished by the use ofauxiliary cathodes or auxiliary anodes, conducting shells, bipolarconductors, and shields. A detailed discussion of the above variouscurrent balancing aids is provided in a publication of Robert H.Rousselot in Metal Finishing Journal at pages 57-63, Mar. 1961. Althoughthese various aids are helpful in current controlling, they have severaldrawbacks. For example, when these aids are used, only an averagecurrent density can be calculated at the cathode. The true currentdensity varies from point to point. The true current density at eachpoint on a cathode is therefore unknown. Further, the cathode areaplated represents only a small portion of the useful area of thecathode, i.e., only the central area of the plated cathode will exhibituniformity of thickness, the remaining areas will be discarded. Thus,inefficient plating is obtained. The current used to plate on thevarious aids is wasted. Chemicals necessary to plate on various platingaids are also wasted. Additionally, in the anode position, its size andshape has to be optimized for each configuration of the cell and foreach configuration of the specific aid used to obtain the best possiblefilm thickness and compositional unifonnity in the small cathode area ofinterest. Further, every time the ionic strength of the plating solutionis changed, the geometry of the electrodes has to be optimized in orderto maintain uniformity of thickness in the deposited film. All of theabove problems are further compounded when it is desired to scale up theplating apparatus from laboratory to production scale. The geometry ofthe cell of all the electrodes and aids, as far as the shape and size ofthe electrodes and of the auxiliary equipment are concerned, has to beoptimized again. The scale up cannot be accomplished by simpledimensional scale up of the cells and electrodes. Thus, a great deal ofexperimentation, of the trial and error variety, must be done every timethe geometry or size of the electroplating cell is changed or when thesize, shape, and spacing of the electrodes are changed.

In an article entitled, Current and Metal Distribution inElectrodeposition, in the publication Plating, Vol. 39, N0. 2, ages -170(1952), there is presented a theoretical discourse on idealized platingbath containers from which metals can be plated having uniformity ofthicknessv The theory disclosed is based primarily upon Ohm s law,neglecting pertinent parameters such as polarization. The article admitsthat while desired results can be obtained theoretically the ideal case,at that time, had not been realized. The article further infers that inorder to obtain the ideal case, one may choose either of twoarrangements for a rectangular container, viz. a viz., A rectangulartank infinitely long in one direction with an anode infinitely far fromthe cathode," or Two very large flat plates immersed in a very largetank so as to be parallel to another. It is readily seen that neither ofthe two above arrangements are practical. Further, it should be notedthat the article attempts to control primary current distribution only,based on its idealization of Ohms law; consequently the article neglectssecondary current distribution problems, which are critical in alloyplating as in the present invention. Thus, while it is indicated thatthe unobtainable ideal case of the article may provide uniformity ofmetal thickness, it nowhere suggests that at the same time uniformity ofcomposition can equally be obtained when plating an alloy such as in thecase of the present invention.

Further, an article entitled, Engineering Design of ElectrochemicalSystems, J. Newman, Industrial & Engineering Chemistry, Vol. 60, No. 4,pages 12-27 (April 1968) discusses the unimportance of Ohms Law,contrary to above article, and clearly demonstrates the importance ofthe secondary current distribution due to proper uniform agitationconditions.

SUMMARY OF THE INVENTION According to one aspect of the invention, animproved electroplating cell is provides. This cell, as is illustratedby the embodiments disclosed herein, includes spaced apart electrodeshaving their edges substantially in contact with a suitable dielectricmaterial, which may constitute the walls of the cell. The cell soprovided requires that the cross-sectional area of the current pathacross the electrodes is substantially the same as the cross-sectionalarea along the length of the electrodes. This results in substantiallyequipotential lines which are parallel to both electrodes, and aconstant current density throughout the whole cathode area. Currentdensity is uniform, well defined, and well known for each point on thecathode. Where constant current plating is used, the anode can be placedat any reasonable distance from the cathode and the plating results arecompletely reproducible from one plating to the other and show excellentlateral uniformity in composition, thickness and magnetic properties.

In scaling up the cell from laboratory to production scale, a simpledimensional scale up of the cell dimensions results in reproducibleplating conditions. Whereas in conventional electrochemical apparatus,dimensional scale up normally cannot be used. Further changes in ionicstrength of the bath in no way affect the plating operation oruniformity of thickness, composition or magnetic properties.

The electroplating cell of this invention can be used for theelectrodeposition of any metal and from any plating bath compositionwhere uniformity of thickness and composition is desired.

Again, in accordance with the principles of the present invention, animproved electroplating method of fabricating magnetic thin film devicesis realized. In the practice of this method, a relatively dilute aqueousbath including nickel-ironcopper is used. The film is plated on a smoothplanar copper substrate cathode in the bath. Plating is accomplished inthe cells of this invention.

Therefore, it is an object of the present invention to provide animproved electroplating cell.

It is a more specific object to provide an improved electroplating cellin which the cross-sectional area of the current path is substantiallythe same as the cross-sectional area along the length of the electrodes.

A further and equally important object is to provide an improvedelectroplating cell in which metal films having uniformity of thickness,composition and magnetic properties can be deposited.

Still another object of this invention is to provide an improvedelectroplating apparatus in which metal films may be plated withuniformity of thickness and composition without the use of currentbalancing aids.

It is still another object of this invention to provide an improvedelectroplating apparatus which may be scaled up from laboratory toproduction size without the need for undue experimentation to optimizethe parameters of the apparatus.

And yet another object of this invention is to provide a method ofelectroplating magnetic films having uniformity of thickness,composition and magnetic properties.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view showingthe electroplating bath cell of this invention.

FIG. 2 is a side sectional view of the structure of FIG. 1 showing thepath of current.

FIG. 3 is a side sectional view of a prior art electroplating bath cellshowing the current path generated therein.

FIG. 4 is a plot comparing uniformity of the thickness of a plated filmin the apparatus of this invention with a like film prepared in a priorart apparatus.

FIG. 5 is a plot comparing the uniformity of composition of a filmcontaining Ni, Fe and Cu prepared in the apparatus of this inventionwith a like film prepared in a prior art apparatus.

FIG. 6 is a plot comparing the magnetic properties, coercive force andanisotropy field of the films used in the plot of FIG. 5.

FIG. 7 is a plot comparing anisotropy dispersion and magnetic skew inthe films used in the plot of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the perspective views ofFIGS. 1 and 2 which show the structure used in one mode of practicingthe present invention, the bath container is designated 10. The walls ofthe container 10 are made from any suitable dielectric material such asglass or a plastic material, e.g., polymethacrylate. On either side ofcontainer 10 there is mounted Helmholtz coils 12 which can be energizedduring the plating of a magnetic film so that the fabricated filmstructure exhibits uniaxial anisotropy. The coils provide a magneticfield of about 40 0e or more. A cathode 14 is in the form of aninsulating board on which there is affixed a conductive sheet orcoating. The upper surface of the conductive sheet is very smooth.Cathode substrate materials which have been found to be satisfactory arerolled copper sheets, evaporated copper, evaporated silver, silver,sputtered gold, said electrolessly deposited silver, copper, nickel orcobalt. Cathode 14 is mounted on support block 16 and is in intimatecontact therewith. Support block 16 is supported on a dielectricmaterial base 18 on the bottom of container 10. Support block 16 is aconductive material which is adapted to receive cathode 14 and whichextends to and is in contact with the walls of container 10. Thisarrangement of cathode 14 and support block 16 effectively extends theedges of cathode 14 to the walls of container 10. Cathode 14 is placedin support block 16 and is substantially flush or level therewith.Alternately, cathode 14 can be made to extend the full width and lengthof container 10 without the aid of support block 16 by simply making thecathode the size of the perimeter of container 10. Contact is made tothe cathode 14 through support post 20, the outside surface of which isinsulated since it is immersed in the electroplating bath. This post 20is connected at a terminal 22 to an electrical current source, notshown. Mounted on a shoulder of post 20 is anode 24. Anode 24 extendssubstantially the full width and length of container 10, so that likecathode 14, its edges are substantially in contact with the insulatingwalls of container 10. Anode 24 is supported in a recess 11 fashioned inthe walls of container 10. The anode 24 can be prepared from aconductive material such as, molybdenum, nickel, platinum and the like.Wound around anode 24 is a wire winding 26 of the same material fromwhich the anode 24 is prepared. Winding 26 is provided to increase thesurface area of anode 24 to at least twice that of the cathode. Thisincreased surface area on anode 24 lowers the current density at theanode thus preventing anodic oxidation deposits which may from time totime fall onto the cathode and interfere with metal plating thereon.Alternately, the surface area of anode 24 can be increased bycorrugating or by grooving the solid anode metal. The electricalconnection to the anode is supplied by a wire connection 28 which leadsto current supply source, not shown.

The bath level during plating is indicated by line 30 with anode 24being in contact or immersed in the bath during the plating operation.The bath is agitated during the plating operation by a motor 32 which isconnected to carrier 36 by linkage designated 34, or any other suitablelinkage. The linkage 34 is designed to conform to the recess 13 ofopposing walls of container 10. Anode 24 is substantially the same sizeas cathode 14. Similarly, linkage 34 is thinly made so that anode 24remains substantially in contact with the walls thereat. When motor 32is energized, the carrier 36 moves the base portion 35 continuously at asubstantially uniform rate in a path back and forth along the length ofthe cathode 14 and just above the surface of cathode 14. As a result, ahomogenization of the bath solution occurs on the surface of cathode 14.The agitating means comprising linkages 36, 34 and the base portion 35is adapted to cause a uniform laminar flow of the bath across surface ofcathode 14 without causing any measurable turbulence thereat. Theagitating means can be fashioned from any nonconductive material such asplastics and the like Turbulence must be avoided since such turbulencecause local non-uniform polarization, thus negates compositionalhomogeneity. To avoid such local turbulence, i.e., below recess 13,agitating means is provided with sharp edges (the lower portion oflinkage 34) so as to provide minimal resistance to the flow of the bath.The base portion 35 is similarly designed to provide minimal resistanceto flow. It is triangular in form with its blunted apex at an anglewhich permits flow thereover with minimal turbulence, while its base isflat. In operation, the agitating causes the bath to flow over the base,and to effect mixing with bulk of the bath at the apex of said base 35by convection. As the mixture passes the apex, the laminar flow isrestored.

Referring again to FIG. 2, the current path, indicated by dash lines 46,is seen to have a cross-sectional area substantially equal to thecross-sectional area of cathode 14 and anode 24, i.e., the currentacross the electrodes 14 and 24 is confined to the boundaries thereofand is not allowed to diverge or spread in its path between saidelectrodes 14 and 24. As a result, the current density is relativelyconstant throughout the whole cathode 14 area. The current density isfound to be relatively uniform and well defined; and the current densityvalue can be predicted at any point on the cathode 14, since they arethe same at any given point thereon. Consequently, films produced in theelectroplating cell of this invention are uniformly thick throughout,and where metal alloys are being plated the metal compositions will alsobe uniform throughout the film 5 thickness.

In contrast, FIG. 3 depicts a prior art electroplating cell, generallydesignated as numeral 1 10, in which current balance aids (guard rings)480 are used to improve current distribution across the cathode 140 andanode 240. It is seen that current (lines) across the electrodes 14 and24 travels in an arcuate path at the edges of the electrodes andgradually begins to travel in parallel lines 460 toward the centralportion of the cathode 140. Thus, only at the central portions ofcathode 140 is there a uniformity of current density. Films prepared inthis cell will have uniformity of thickness at the central portionsthereof; consequently, efficient use of the film cannot be had becausethe outer portions are nonuniform in thickness and must be discarded.This is especially true where magnetic films are plated, sincenonunifonnity in the films thickness and/or film composition alsoresults in nonuniformity of magnetic properties in the film.

To better illustrate the invention, several magnetic films were platedonto the cathode in the apparatus of this invention and compared withmagnetic films similarly plated in the prior art apparatus shown in FIG.3. The films were plated from baths having the following compositions:

Bath Number 1 2 3 4 gJl 5-] all 841 Triton x- 100 0.8 0.6 l 0.9

S ulfamic Acid 1. 1.0 W A Succharin .0 0.4 KNacJ-nm- 411,0 7.5 7.5NiSO,-6H,0 15.0 15,0 F=so 7H,o 2.0 2.5 CuSO 5 11,0 1.25 Nick-611,0 109FeCl,4H,O 3.88 11,50, 12.5 Na Lauryl Sulfate 0.2 cu No, 100 Sulfuricacid (conc.) 30 ml./l Formic acid (conc.) 4O rnL/l Acetic acid(anhydrous) 20 ml./l

I Platings from baths 1 and 2 were made using the pulse platingtechnique described in US. Pat. application Ser. No.

573,417, now US. Pat. No. 3,480,522 to .lamm M. lBrownlow, and havingthe same assignee as this application. The description oil the pulseplating technique described in the above stated application isincorporated herein.

The films are plated at constant current without agitation for 10 to 15second intervals. They can also be plated with shaped current pulses.After the plating intervals are completed, the bath is agitated and isallowed to come to rest for about 15 to 60 seconds. This sequence ofsteps is repeated until the desired film thickness is attained. Allplatings were performed in a magnetic field of 40 oersteds and a bathtemperature of 20 C. The films were plated on cathodes having an area of3 X 3 inches. Films having thicknesses of from 1,000 A to l,800 A wereplated.

Plating from baths 3 and 4 were made using well-known continuous platingtechniques, as opposed to the above described pulse plating technique.The solution was agitated continuously during the plating process.

At the completion of the plating operation, the magnetic films weretested for uniformity of composition, thickness and magnetic properties.Films from baths 1 and 2 were plated on 800 A of silver evaporated on 15mil thick 3 X 3 inch glass substrates. Measurements of thickness,composition and of magnetic properties coercive force, anisotropy field,anisotropy dispersion, and magnetic skew were made on the 3 inches by 3inches plates along the diagonal from corner to corner at convenientintervals. Exactly the same spot was used to take all the measurements.At least 10 spots were examined along the diagonal.

The composition and thickness were evaluated using 1 mm. X 3 mm. spot inconnection with the X-ray fluorescence technique. Characteristic Kradiation of Ni, Fe and Cu was monitored from which composition andthickness were determined.

Magnetic properties, H H a and )3, were measured using the Kerrmagneto-optic technique which is well known in the art. A spot 3 mm. X 3mm. in size was examined in each case. The spot selected was always thesame spot which was previously examined using the x-ray fluorescencetechnique for film thickness and alloy composition.

Illustrative of uniformity of film thickness obtained from thisinvention is the plot of film thickness as measured along the diagonalof the film shown in FIG. 4. It is seen that the profile of thethickness along the diagonal of the film plated in the cell of thisinvention (represented by the heavily drawn line) is relatively uniformthroughout the film, while the thickness of films obtained from theprior art cell (shown by the curve labeled prior art cell) is nonuniformin character.

Referring to FIG. 5, there is shown a comparison plot of the Ni, Fe andCu composition measurements taken along the diagonals the plated filmsprepared by this invention and by the prior art. The plot isrepresentative of measurements made on a large number of films. Theheavily drawn lines are indicative of the relatively uniformcompositions obtainable only in this invention. The more finely drawnlines are indicative of the nonuniform compositions obtained by theprior art.

Magnetic properties of the films prepared by this invention and by theprior art are shown in FIGS. 6 and 7. The heavily drawn lines are againrepresentative of the uniformity of magnetic properties of films platedby this invention, and the finely drawn lines show the nonuniformityexhibited by films prepared by the prior art.

In summary, an apparatus for plating metal films having highly uniformthickness, composition and magnetic properties throughout the film hasbeen devised. The apparatus is characterized by having an anode and acathode arranged in a bath container such that the edges of the anodeand cathode are substantially in contact with a dielectric material. Theelectrodes so arranged prohibit spreading of the current in theelectrolyte along its path across the plates. Therefore, equipotentiallines are formed parallel to both electrodes, current density is uniformand constant throughout the whole cathode area.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be underunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:

1. An improved apparatus for electrodepositing metal films havingsubstantially uniformity of thickness and composition having incombination a plating bath container to contain a plating bath disposedtherein, having all four of its sides and base, fashioned from adielectric material;

A pair of electrode means spaced apart in said bath container and insubstantial contact with the walls thereof, being mounted within saidcontainer and said plating bath, one of said electrode means being aconductive block supported on the dielectric base of said container andbeing adaptable to receive a conductive substrate member for completinga current path across said electrode means;

non-conductive agitating means disposed in said container for providinguniform laminar flow of said bath across the surface of one of saidelectrode means;

said non-conductive agitating means being constructed so as to provideminimal resistance to the flow of said bath, thereby preventingturbulence therein; and

means for applying a current across said electrode means whereby acurrent path having a cross-sectional area which is substantially thesame as the cross-sectional area along the length of said electrodemeans is provided.

2. An apparatus according to claim 1 wherein there is added supportmeans for maintaining said electrodes in spaced apart relation.

3. An apparatus for electroplating metal films according to claim 1wherein the sides of said non-conductive agitation means have sharpedges and the base portion thereof is triangular in shape so as toprovide minimal resistance to the flow of said bath during agitationthereof.

4. An apparatus for electroplating metal films according to claim 1wherein opposite walls of said container are recessed so as to supportone of said electrodes.

5. An apparatus for electroplating metal films according to claim 1including magnetic field generating means disposed outside of saidcontainer to provide a magnetic field of about 40 cc to therebyestablish plane orientation in an electrodeposited magnetic film.

k l a t I!

2. An apparatus according to claim 1 wherein there is added supportmeans for maintaining said electrodes in spaced apart relation.
 3. Anapparatus for electroplating metal films according to claim 1 whereinthe sides of said non-conductive agitation means have sharp edges andthe base portion thereof is triangular in shape so as to provide minimalresistance to the flow of said bath during agitation thereof.
 4. Anapparatus for electroplating metal films according to claim 1 whereinopposite walls of said container are recessed so as to support one ofsaid electrodes.
 5. An apparatus for electroplating metal filmsaccording to claim 1 including magnetic field generating means disposedoutside of said container to provide a magnetic field of about 40 oe tothereby establish plane orientation in an electrodeposited magneticfilm.